The invention relates to a method and an apparatus for treating a fibrous web. Particularly, but not solely, the invention relates to a method and an apparatus for sizing and calendering paper and board.
The sizing and calendering of paper and board utilize a separate size press and calender. The size press can employ, inter alia, pond sizing technology or film transfer technology. The separate apparatuses required by the arrangement make the apparatus expensive and the apparatus consumes plenty of space in the machine line.
For moistening and calendering paper and board, it is possible to use a wetstack calender where the set of rolls of the calender is provided with one or several water boxes.
Patent specification U.S. Pat. No. 5,938,895 describes a calender where water is dosed via the lip of a water box onto the surface of a calender roll.
Patent specification U.S. Pat. No. 6,280,574 describes a calender where water is dosed as a film directly onto a fibrous web before a calendering nip and water is scraped from the fibrous web before the calendering nip to form a thin water film.
Patent specification EP0670004B1 describes a method and an apparatus for coating a traveling material web. Liquid is indirectly dosed onto the web by spraying the liquid on a roll shell from where the liquid is transferred onto the web.
Published application DE102008021541A1 describes a method in which treatment medium is sprayed onto an unsupported fibrous web in a spraying chamber and the web is guided within the spraying chamber to a press nip.
When manufacturing board, such as packing board and fluting board, strength required by the end-product can be provided by using virgin fibrous material. The use of recycled fiber weakens board strength. When manufacturing board of recycled fibers, the board can be treated on the size press where starch is added to the board by surface sizing. Using starch aims at compensating the weakening of board strength due to the use of recycled fiber.
However, a limitation of the traditional size press technology is the dosing of large starch quantities or dosing a sufficient starch quantity in recent high-speed board machines. In the traditional size press, only a specific amount of wet film can be spread onto the roll without a pond forming in the nip. As ponds form in the nip, starch starts to splash and the additional dosing of starch onto the roll does not necessarily increase the amount of starch in the web.